Cody C. Farnell

An overview of the Nuclear Electric Xenon Ion System (NEXIS) program

NASA is investigating high power, high specific impulse propulsion technologies that could enable ambitious flights such as multi-body rendezvous missions, outer planet orbiters and interstellar precursor missions. The requirements for these missions are much more demanding than those for state-of-the-art solar-powered ion propulsion applications. The purpose of the NEXIS program is to develop advanced ion thruster technologies that satisfy the requirements for high power, high specific impulse operation, high efficiency and long thruster life. The nominal design point for the NEXIS thruster is 20 kWe at a specific impulse of 7500 s with an efficiency over 78% and a xenon throughput capability of greater than 2000 kg. These performance and throughput goals will be achieved by applying a combination of advanced technologies including a large discharge chamber, erosion resistant carbon-carbon grids, an advanced reservoir hollow cathode and techniques for increasing propellant efficiency such as grid masking and accelerator grid aperture diameter tailoring. This paper provides an overview of the challenges associated with these requirements and how they are being addressed in the NEXIS program.

NEXT Ion Optics Simulation via ffx

Simulations of the erosion processes for two proposed sets of ion thruster grids for the NEXT project are presented. Structural failure and electron backstreaming due to accel grid erosion are discussed as two possible failure mechanisms of these grid sets. The TAG grid set is shown to outperform the NSTAR grid set both in terms of margin against electron backstreaming and accel grid mass loss at the primary operating condition studied. An investigation into the possibility of reducing the accel grid voltage magnitude for the TAG grid set showed improved propellant throughput capability. Results of erosion simulations predicting propellant throughput capability for the TAG grid set are presented for a range of NEXT operating conditions.

Magnetic filter type plasma source for ground-based simulation of low earth orbit environment

Simulation of the low Earth orbit (LEO) plasma environment in ground-based vacuum facilities is important for studies of spacecraft interaction with ionospheric plasmas. In this paper we describe the design and performance of a magnetic filter-equipped plasma source. Experimental data collected in the expanding plasma downstream of the source suggest it is a good candidate for use as a LEO plasma simulator in that the expanding plasma has a very low electron temperature and contains streaming ions—the plasma environment encountered by satellites in LEO. Adjustable plasma source operating conditions of flow rate, discharge current and discharge voltage enable production of plasma electron temperatures over the range from 0.17 to 0.35 eV and streaming ion energies over the range from 1 to 4 eV.

Numerical Simulation of HiPEP Ion Optics

The ffx code was used to investigate the lifetime and propellant throughput capability of the High Power Electric Propulsion (HiPEP) thruster ion optics as part of NASA’s Project Prometheus. Erosion predictions are presented as a function of beamlet current, accel grid voltage, and propellant utilization efficiency. These predictions were then compared to the lifetime goals of the thruster, and for nominal operating conditions, the ffx code indicates that the HiPEP thruster will have propellant throughput capability of 100 kg/kW and lifetimes in excess of 135 khrs. In addition to lifetime assessment, a detailed study was completed where grid design parameters were varied in a systematic manner to determine their effects on beamlet current limitations and electron backstreaming margins. The parameters that were varied included the net and total accelerating voltage, grid spacing, aperture center-to-center distance, accel grid thickness, screen grid thickness, discharge voltage, and accel grid aperture diameter.

Comparison of hollow cathode discharge plasma configurations

Hollow cathodes used in plasma contactor and electric propulsion devices provide electrons for sustaining plasma discharges and enabling plasma bridge neutralization. Life tests show erosion on hollow cathodes exposed to the plasma environment produced in the region downstream of these devices. To explain the observed erosion, plasma flow field measurements are presented for hollow cathode generated plasmas using both directly immersed probes and remotely located plasma diagnostics. Measurements on two cathode discharge configurations are presented: (1) an open, no magnetic field configuration and (2) a setup simulating the discharge chamber environment of an ion thruster. In the open cathode configuration, large amplitude plasma potential oscillations, ranging from 20 to 85 V within a 34 V discharge, were observed using a fast response emissive probe. These oscillations were observed over a dc potential profile that included a well-defined potential hill structure. A remotely located electrostatic analyzer (ESA) was used to measure the energy of ions produced within the plasma, and energies were detected that met, and in some cases exceeded, the peak oscillatory plasma potentials detected by the emissive probe. In the ion thruster discharge chamber configuration, plasma potentials from the emissive probe again agreed with ion energies recorded by the remotely located ESA; however, much lower ion energies were detected compared with the open configuration. A simplified ion-transit model that uses temporal and spatial plasma property measurements is presented and used to predict far-field plasma streaming properties. Comparisons between the model and remote measurements are presented.

Quartz crystal microbalance-based system for high-sensitivity differential sputter yield measurements

We present a quartz crystal microbalance-based system for high sensitivity differential sputter yield measurements of different target materials due to ion bombardment. The differential sputter yields can be integrated to find total yields. Possible ion beam conditions include ion energies in the range of 30-350 eV and incidence angles of 0 degrees-70 degrees from normal. A four-grid ion optics system is used to achieve a collimated ion beam at low energy (<100 eV) and a two-grid ion optics is used for higher energies (up to 750 eV). A complementary weight loss approach is also used to measure total sputter yields. Validation experiments are presented that confirm high sensitivity and accuracy of sputter yield measurements.

Ion Thruster Grid Design Using an Evolutionary Algorithm

An evolutionary algorithm was used to optimize the geometry and accelerator grid voltage of an ion thruster grid set with regard to maximizing impulse per unit area, essentially equivalent to maximizing propellant throughput capability per unit area. Grid operating conditions, including a net accelerating voltage of 1800 V and current density of 4.0 mA/cm 2 , corresponded to high-power operation of NASAs evolutionary xenon thruster. The evolutionary-algorithm-derived grid set had a predicted lifetime nearly twice that of the NASAs evolutionary xenon thruster grid set, primarily the result of a lower accelerator grid voltage magnitude.

Ion Emissive Membranes for Propulsion Applications

Experiments show electrostatic thrusters with components such as the discharge chamber or acceleration channel, solenoid or permanent magnets, hollow cathode, and keeper can be replaced by a simple, propellant‐selective, solid‐state, ion‐conducting membrane (Wilbur et al., 2007; Wilbur, Wilson, and Williams, 2005). In addition, analyzes show these membranes can be shaped, structured, and assembled into integrated thruster systems that will operate at much greater thrust densities and thruster efficiencies than those for state‐of‐the‐art, Hall and ion thrusters (Wilbur, Farnell, and Williams, 2005). The implications of these findings are revolutionary and promise an electrostatic propulsion system much less massive, more reliable, and less costly than ion and Hall thruster systems as they can be fabricated readily using traditional ceramic manufacturing techniques. The status of the Emissive Membrane Ion Thruster (EMIT) concept is described and recent measurements are used to estimate the performance of a ...

Non-invasive Hall current distribution measurement in a Hall effect thruster

A means is presented to determine the Hall current density distribution in a closed drift thruster by remotely measuring the magnetic field and solving the inverse problem for the current density. The magnetic field was measured by employing an array of eight tunneling magnetoresistive (TMR) sensors capable of milligauss sensitivity when placed in a high background field. The array was positioned just outside the thruster channel on a 1.5 kW Hall thruster equipped with a center-mounted hollow cathode. In the sensor array location, the static magnetic field is approximately 30 G, which is within the linear operating range of the TMR sensors. Furthermore, the induced field at this distance is approximately tens of milligauss, which is within the sensitivity range of the TMR sensors. Because of the nature of the inverse problem, the induced-field measurements do not provide the Hall current density by a simple inversion; however, a Tikhonov regularization of the induced field does provide the current density distributions. These distributions are shown as a function of time in contour plots. The measured ratios between the average Hall current and the average discharge current ranged from 6.1 to 7.3 over a range of operating conditions from 1.3 kW to 2.2 kW. The temporal inverse solution at 1.5 kW exhibited a breathing mode frequency of 24 kHz, which was in agreement with temporal measurements of the discharge current.

Superlattice-based Thermoelectric Materials Fabricated using Ion Beam Deposition

Experimental results are presented of thermoelectric materials, specifically two-dimensional quantum well confinement superlattices, formed by ion beam sputter deposition methods. Applications of these thermoelectric devices include nearly any system that generates heat including waste heat. The targeted applications of this research include harvesting of waste heat from stand-alone generator systems and automobiles. Thermoelectric generator modules based on an in-plane orientation of nano-scale, thin-film, superlattices have demonstrated very high performance and are appropriate for a wide range of waste heat recovery applications. In this project, a fast, ion-beam-based deposition process was developed for producing Si/SiC (n-type) and B4C/B9C (p-type) superlattices. The deposition process uses low-cost powder targets, a simplified substrate holder with embedded heater, a QCM deposition rate monitor, and stepper-motor-controlled masks. Deposition times for individual layers are shown to be significantly shorter than those achieved in magnetron-based systems. As an example of the speed of the process, a 10-nm thick Si layer can be deposited in as little as 20 sec while a SiC layer can be deposited in less than 100 sec. Electrical resistivities, thermal conductivities and Seebeck coefficients are reported for the deposited films as well as their respective non-dimensional figures of merit (zT). Figures of merit (zT) approaching 8 and 25 at modest temperatures of ~600 K were calculated for the n and p type materials, respectively. These measurements are made in-plane where enhanced Seebeck values and reduced electrical resistivities have also been reported in the literature. A method for directly measuring thermal conductivity in the plane of the superlattice is described that uses MEMs-based SiN cantilevers. Results are presented for various deposition variables, including film thickness, temperature, deposition energy, and material. Scanning white light interferometry (SWLI) and scanning electron microscopy (SEM) were used to characterize film thickness. In addition to the experimental effort, an analysis was performed to predict the performance of a thermoelectric module fabricated with the superlattice films deposited on ceramic substrates. Thermal efficiencies approaching 15% are predicted for modest cold and hot side temperatures. Thermal conduction through the substrate was found to be the largest factor limiting the performance of the modeled thermoelectric module.